In order to enhance printing resolution and output efficacy, most xerographic devices use a toner composition having sphere or sphere-like particles with a high degree of luster, small particle diameters, and a narrow distribution of the particle diameters to produce their outputs. The smaller the particle diameters and the narrower the particle diameter distribution of the toner particles, the better the printing resolution and uniformity of the outputs are. Additionally, an excessive amount of a luster releasing agent is usually added in the toner composition to improve the printing glossiness. Sphere or sphere-like toner particles have better particle transferring efficiency and flowability. The output efficacy of a xerographic device depends upon the hot-fixing ability of the toner particles. Therefore, a preferable resin used to produce a toner composition will be the resin having a strong elasticity and a low melting temperature, such as polyester.
Generally, toner production processes can be classified as a grinding method and a non-grinding method. Conventional toner particles are manufactured by the grinding method, where the resin used must be brittle. Resins with lower molecular weights are usually chosen for this purpose. However, for such resins, when grinding, flakes and dust powders are formed, which may contaminate the carrier of static developer and make the carrier functionless. For the low molecular weight resins, the weakness in its mechanical strength and melting elasticity may result in toner particles sticking on surfaces of printer parts, such as developer roller, wipe blade, organic photoconductor (OPC) drum or fuser roller, etc, thereby causing printing defects, for example, vertical scratches and/or hot-offset. The thermal properties of the low molecular weight resins, such as glass transferring temperature, are difficult to control. Besides, the toner particles manufactured according to the conventional grinding method vary greatly in size, which leads to the reduction of the production rate of toner particles, and therefore, increase of the production cost. When in use, the irregularity of sizes of the toner particles reduces the printing quality of printing image. Because the size of toner particles is hard to control during grinding, a lot of tiny toner particles are produced. The tiny toner particles can cumulate inside the developer of printing devices, which may reduce the lifetime of the developer.
To overcome the above mentioned disadvantages, a non-grinding method has been introduced for producing toner particles. Toner particles according to the non-grinding method are uniform in shape and size, and sphere or sphere-like. Non-brittleness resins can also be used for toner production by the non-grinding method. Additionally, the non-grinding method can be used for the thermal sensitive toner additives. At present, a non-grinding, wet aggregation-coalescence method is frequently adapted for toner production. Based on the way of aggregation-coalescence of particles, the method is categorized into a suspension method and an emulsion-aggregation method.
The so-called suspension method includes two methods. One is a suspension polymerization method. The suspension polymerization method comprises mixing vinyl monomer, a pigment, and additives to form an oil phase; suspending the oil phase into an aqueous phase containing a dispersing agent by a homogenizer; and adding a radical polymerization initiator to polymerize the vinyl monomer to obtain spherical polymer particles. Note that, according to the method, resins are limited to vinyl polymer resulting from radical polymerization. The polymerizing process takes longer and may be disturbed by additives. Additionally, monomer residues in polymerization are not easily removed.
The other suspension method is a resin dispersion method. The method includes the processes of dissolving resin in a solvent that is immiscible with water; suspending the resin solution in an aqueous phase that contains dispersing agent by a homogenizer; allowing the resin particles to aggregate and coalesce to a desired size; and removing the solvent and obtaining resin particles. The method is suitable for any types of resins. However, a high-speed and high shearing homogenizer is necessary for implementing the method, which consumes a lot of energy. Additionally, resin particles according to the method may not be uniform in size since the resin colloidal particle are dispersed a mechanical force.
For the emulsion-aggregation method, a resin emulsion is formed, and aggregation and coalescence occur in the resin emulsion to form the desired size of resin particles. Based on the process of forming the resin emulsion, the emulsion-aggregation method can be classified into three categories: The first category is an emulsion polymerization method, which includes the steps of mixing an oil phase having a vinyl monomer with an aqueous phase having an emulsion agent, dispersing it with an emulsion homogenizer, and adding an initiator to trigger polymerization to form a resin emulsion. Similar to the suspension polymerization method, the method is suitable for limited types of resins; and there are also drawbacks of a long polymerization time and monomer residues.
The second category is direct emulsification, which includes dissolving a resin in a solvent to form an oil phase, mixing the oil phase with an aqueous solution having an emulsion agent, then, directly dispersing it with an emulsion homogenizer to form a resin emulsion. The method is suitable for any types of resins. Similar to the resin dispersion method, the process has the high energy consumption and the need of removal of solvents.
The third category is indirect emulsification, which comprises the steps of dissolving resin in a solvent to form an oil phase, adding an aqueous solution containing a emulsion dispersing agent to the oil phase, triggering phase changes by altering the volume ratio of the oil and aqueous phases so as to obtain a resin emulsion of high stability and uniform particle distribution. The method has been reported, for examples, in U.S. Pat. Nos. 5,928,831, 6,001,528, and 6,171,743, and Japan Pat. Nos. 61-91666, 63-25664 and 04-303849. According to the method, there is no limit to resins to be used and no need of an energy consumed homogenizer. However, for highly viscous resins, such as resins of high molecular weights and high solid content, phase changes of the emulsion may be interrupted, thereby causing unstable suspension solution with large resin particles to be formed or a phase separation. Accordingly, diameters and a diameter distribution of resin particles to be produced are uncontrollable. This is because, during phase changes, the viscosity ratio of the solution will increase suddenly, which makes the viscosity ratio of the resin emulsion too high. For an operation of at a low rotating speed, a highly viscous resin can not be effectively cut into emulsion microparticles. Therefore, a suspension solution with large and non-uniformly sized particles can be formed, and the resin may even precipitate to form chunk pieces of the resin in certain cases. As a matter of course, the indirect emulsification method is only suitable for resin solutions having low viscosity ratio, such as low molecular weight or low solid content resin solutions. As discussed above, the use of a low molecular weight resin to producing toner particles has disadvantages.
U.S. Pat. No. 5,691,095 discloses an improvement of the method by using self-emulsifiable resin, which is suitable for a high molecular weight and solid content resin solution. According to the improved method, a dispersibly and stably hydrophilic base is introduced into a resin to make the resin self-emulsifiable, dispersible and stable. However, the resin is specially made, this may increase the production cost. Additionally, the hydrophilic base in the resin may adsorb to water in the air, which may result in negative effects on the environment and the toner electrostatic property.
As discussed above, the third category of the indirect emulsification method requires no limitation in material selection and manufacturing facilities. However, there are several questions the method is unable to answer: (1). for an operation at a low rotating speed, how to form uniform, stable, and dispersed emulsion particles from highly viscous resins with high molecular weights and high solid content, so as to increase productivity; (2). how to quickly remove the solvent from liquidized resin particles, and meanwhile to remain the dispersion stability of emulsion particles unchanged, so as to reduce production time; (3). how to precisely control shapes, diameters, and a diameter-distribution of toner particles, so as to improve the printing quality.
The printing quality of laser printers is greatly affected by the shapes, diameters, and diameter-distribution of toner particles. This is especially true to grayscale and full color printings that require high quality of image printing. Uniformly sized toner particles can produce smooth images and good color concentration while consuming less toner. Thin and smooth layer of toner make better transparency properties of images and the texture of images looks similar to lithograph. Moreover, because of the thin layer of toner on images, the efficacy of heat fixing for a printer is improved, leading to reducing energy consumption and increasing print speed.
It is known that the shapes of toner particles play an important role in tribocharging, powder flowability, cleanability, accumulation density, uniformity of the toner particles. The rougher the surface (e.g., conventional toners) of a toner particle is, the greater the surface friction is, and the better tribocharging and cleanability properties it bears. However, the rough surface of toner particle causes poor powder flowability, accumulation density, and uniformity. On the other hand, the more uniform shape and narrow diameter distribution toner particles have, the more uniform tribocharging, and the higher toner transfer efficiency, and the better accumulation density and uniformity the printing images show. However, the toner particles have poor tribocharging and cleanability properties. Therefore, a key process in toner productions is to manufacture toner particles having their shapes controllable.
For toner particles having different diameters, different particles obtain different tribocharging during operation of a toner cartridge. The difference makes small toner particles bearing higher electrostatic charges, which result in poor printing images and lack of toner powder transferring when printing. Moreover, non-uniform distributed electric charges on toner particles may cause printing offset, backgrounding and smear edges.
Briefly, in the non-grinding methods, resins to be used are limited, for example, it may not be efficient for highly viscous resins of high molecular weights and high solid content. Additionally, toner production processes are highly energy consumed. For toner particles manufactured according to the conventional methods, their shapes, diameters, diameter distribution are not controllable.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.